17. Vegetation and Climate of the Northwest Coast of North America during the Last 500 K.y.: High-resolution Pollen Evidence from the Northern California Margin

Pollen analyses of sediments from Holes 1019C, 1019E, 1020C, and 1020D as well as piston Core EW9504-17 provide continuous, chronostratigraphically controlled proxy vegetation and climate data for coastal northwest North America for the last ~500 k.y. Systematic changes in the representation of the diagnostic components of northern California plant assemblages clearly show orbital-scale variations. Interglacials are all marked by an abrupt increase in alder followed by expansion of lowland oak woodland and redwood forests. Glacials are dominated by montane forest and woodland assemblages. This sequence reflects large-scale climatic controls (e.g., orbital-scale variation in insolation and Northern Hemisphere ice sheets) in western North America over the last five glacial cycles. Regional climatic control (variations in sea-surface conditions) is implied by the differential development of xeric oak and mesic redwood communities. 1Lyle, M., Koizumi, I., Richter, C., and Moore, T.C., Jr. (Eds.), 2000. Proc. ODP, Sci. Results, 167: College Station TX (Ocean Drilling Program). 2Lamont-Doherty Geological Observatory, Columbia Univeristy, Palisades NY 10987, USA. (Present address: Heusser and Heusser, Inc., 100 Clinton Road, Tuxedo NY 10987, USA.) [email protected] 3Center for Geophysical Investigation of the Shallow Subsurface, Boise State University, Boise ID 83725, USA. 4College of Ocean and Atmospheric Science, Oregon State University, Corvallis OR 97331, USA. INTRODUCTION It has been established that pollen deposited in marine sediments on continental margins provides terrestrial vegetation and climate records that are directly correlated with global chronostratigraphic events (Dupont, 1992; Heusser and van de Geer, 1994; Van Campo et al., 1982). Interpretations of past vegetation and climate of western North America from pollen deposited offshore are based on comparisons of downcore pollen spectra with modern marine and terrestrial pollen spectra (e.g., pollen assemblages from marine and terrestrial sediment traps and surface samples), vegetation, and climate (Heusser and Balsam, 1977; Heusser, 1988). Previous studies showed that Milankovitch-scale variations in vegetation of coastal Washington and Southern California reconstructed from pollen deposited in the northeast Pacific Ocean during the last glacial cycle were similar to those inferred from onshore pollen data ascribed to the same time (Heusser and Florer, 1973; Heusser, 1995). To extend these studies in space and time, we present preliminary results from pollen analyses of sediments deposited off Northern California during the last ~500 k.y. Pollen from piston cores taken on the continental margin of western North America between 32°N and 43°N also showed systematically related short-term changes superposed on Milankovitch-scale oscillations of the past 60 k.y. During oxygen isotope Stage 3, for example, brief warming events in California and Oregon appear correlative with interstadial events in waters offshore (Heusser, 1998). Here we extend high-resolution pollen analyses of sediments deposited on the northern California margin over the last ~140 k.y.